The duration for which a magnet remains magnetized after being removed from the fridge depends on various factors such as the type of magnet, the strength of the magnet, the conditions it's exposed to, and the materials it interacts with.
Permanent Magnets: Permanent magnets, like the ones often used on refrigerator doors, are made from materials with high magnetic retentivity. These magnets can retain their magnetization for a very long time, potentially years or even decades, without significant loss of strength. As long as they are not subjected to extreme heat or strong magnetic fields, they will remain magnetized for an extended period.
Temporary Magnets: Some magnets, like those made from soft iron, are temporary magnets. They can be easily magnetized but lose their magnetization quickly once the external magnetic field is removed. These magnets are not commonly used in everyday applications like fridge magnets.
Demagnetization over Time: All magnets, even permanent magnets, can experience a gradual loss of magnetization over time due to various factors like temperature changes, exposure to other magnetic fields, and mechanical shock. However, this process is usually slow for high-quality permanent magnets.
Elevated Temperature: High temperatures can weaken the magnetization of permanent magnets. If a fridge magnet, for example, is exposed to excessive heat, it may lose some of its magnetism more quickly.
In general, the magnets used in everyday applications like fridge magnets tend to remain magnetized for a very long time, often many years, unless subjected to adverse conditions. The act of being on a fridge door does not significantly affect their magnetization over time.